Oil and gas transmission pipelines of large diameter (12"-60") are usually coated and then buried before being used for transportation of fluids. The coatings serve to reduce corrosion caused by the environment in which the pipeline operates. Various forms of coating materials have been used over the years. Coal tar products were and are well known as coating materials and, more recently, polyethylene tape layered coatings have been used.
The coating may be put on the pipe after it has been welded together in sections and before the welded line is buried. The coating process is usually continuous. In an alternate case the pipe sections are delivered to the site already shop coated except for 1'-2' on each end. A second coating is applied to cover the previously uncoated ends of each section after the welding and before the line is buried.
In recent developments several pipeline operators have experienced underground failures of old coatings. These failures are most commonly attributed to disbondments between parts of the coating and the pipe. Despite the continuous use of cathodic protection, the disbondment sites are subjected to pitting corrosion and to stress corrosion cracking (SCC) and, in severe cases, pipe failures have occurred under pressure. The situation has prompted many operators to initiate coating rehabilitation projects. Almost all SCC cases have been encountered in lines in the ground for 10 years or more.
Various devices and techniques have been developed for the purpose of facilitating the rehabilitation of a pipeline coating. The most common technique employs a self-propelled device fitted around the pipe which continuously cuts, scrapes and brushes the coating with steel knives and brushes. This method does remove some of the oldest coal tar coatings fairly well but performs unsatisfactorily on the polyethylene tape layered coatings of more recent vintage. The process leaves adhesive and tape residue. The knives utilized in this process can seriously damage the pipe surface. Devices of this type have been around for approximately 20 years.